The present invention relates to a moving body detecting apparatus for detecting a change in a magnetic field in accordance with movement of a magnetic moving body, particularly relates to a moving body detecting apparatus which is preferable by being used in a case of detecting information of rotating a soft magnetic gear used in an industrial machining tool, an engine of an automobile or the like.
In a related art, there is known a moving body detecting apparatus used for a rotation sensor or the like using an intensity detecting type giant magneto resistive element (hereinafter, intensity detecting type GMR element).
There is known a rotation sensor for detecting rotation of a magnetic moving body (magnetically detected body) of a soft magnetic gear or the like which is arranged with magneto sensitive elements opposed to a magnetic moving body separately at two regions. An interval of arranging the magneto sensitive elements constitute an interval adapted to a projection-to-projection pitch of the magnetic moving body (with respect to a projection-to-projection pitch=P of a gear, a magneto sensitive element aligning interval L=P/2 is regarded to be optimum). Further, when the magnetic moving body is rotated, a signal in correspondence with the recess and projections is outputted.
Particularly, when a magneto resistive element is used as a magneto sensitive element, there is proposed a constitution of including 2 pieces of magneto resistive elements at each of the above-described two regions and a total of 4 pieces of the magneto resistive elements are integrated to a Wheatstone bridge circuit in JP-A-9-329462.
According to the patent reference, when there is a projected portion of a gear at one of the magneto sensitive regions, a recessed portion of the gear is disposed at other of the magneto sensitive regions and therefore, outputs of the magneto resistive elements are inversely polarized. An output of 4 times as much as that of one element is provided by outputting a difference thereof by integrating the magneto resistive elements to the Wheatstone bridge circuit.
Meanwhile, the magneto resistive element used in the rotation sensor of the related art is of a magnetic field intensity sensing type and is provided with a property that a resistance value thereof becomes maximum when an external magnetic field is 0 and when the external magnetic field is increased, the resistance value is reduced. FIG. 1A schematically shows an arrangement of a magneto sensitive element, a bias magnet and a soft magnetic gear in a case of a related art in which the magnetosenstive elements of the magnetic field intensity sensing type are arranged separately at two regions, an interval of arranging magneto sensitive regions is designated by notation L and a projection-to-projection pitch of the gear is designated by notation P. 2 pieces of the magneto resistive elements 'are arranged at each of the magneto sensitive element regions.
FIG. 1B; FIG. 1C show signal outputs and a differential output from respective elements (detected outputs when a Wheatstone bridge circuit is integrated by a total of 4 pieces of magneto sensitive elements) when the elements are optimally arranged in the related art, signal output phases of the elements of the two regions are shifted from each other by 180° as shown by FIG. 1B and therefore, the differential output of FIG. 1C becomes a maximum.
FIG. 1D, FIG. 1E are for explaining a problem of the related art (when deviated from the optimum element arrangement), showing signal outputs and a differential output from the respective elements when L>P/2, the shift of the signal output phases of the elements of the two regions is reduced as shown by FIG. 1D and therefore, the differential output of FIG. 1E is reduced.
As has been explained in reference to FIGS. 1A through 1E, the magneto sensitive elements used in the rotation sensor of the related art is of the magnetic field intensity sensing type and therefore, there poses a problem that it is necessary to align the two region at an interval L=P/2 relative to the projection-to-projection pitch (=P) of the gear in order to provide the optimum change in the output and when the projection-to-projection pitch (=P) of the gear and the interval L between the two regions are brought into a relationship of L>P/2, a shift of phases of output signals of two regions is reduced and an optimum differential signal output from the Wheatstone bridge circuit is not provided (an amplitude of the differential output is reduced).
Further, there is proposed other moving body detecting apparatus in JP-A-9-329461. According to the detecting apparatus, in a constitution of arranging intensity detecting type GMR elements between a gear as a magnetic moving body and a magnet, even in a case of an intensity detecting type GMR element having a hysteresis, it is determined that a range of changing a resistance value becomes symmetric by shifting to arrange a center of a magneto sensitive face and a center of the magnet to thereby provide a waveform in correspondence with edges of recessses and projections of the gear. A waveform at that occasion is shown in FIG. 2A. FIG. 2A shows a detected waveform of the intensity detecting type GMR element in correspondence with a projected portion of the gear as the magnetic moving body, and notations Vta, Vtb designate threshold voltages when the detected waveform of the intensity detecting type GMR is shaped. FIG. 2B shows a rectangular waveform output after shaping the waveform.
FIG.3 shows a magnetic property of the intensity detecting type GMR element having a hysteresis characteristic. According to the apparatus of JP-A-9-329461, when the Wheatstone bridge circuit is integrated by using two pairs of intensity detecting type GMR elements, a detected waveform in a positive direction and a detected waveform in a negative direction are set to provide peaks of the same degree such that one of the paired intensity detecting type GMR elements constitutes an operating point on a right side of a curve and other thereof constitutes an operating point on a left side of the curve to constitute the operating points symmetric with each other.
Meanwhile, the GMR element used in the above-described detecting apparatus is of the magnetic field intensity sensing type (multilayer film type) and is provided with the hysteresis as shown by the characteristic diagram of FIG. 3. A detecting signal sampled from the Wheatstone bridge circuit is constituted by a waveform having peaks on upper and lower sides as shown by FIG. 2A. Notation Vta of FIG. 2A designates a rise threshold voltage and notation Vtb designates a fall threshold voltage. It is devised that by providing a width in the threshold voltage in this way, even when the detected waveform is more or less moved in an up and down direction by a temperature drift, the detected waveform traverses the threshold voltage. Meanwhile, in a relationship between a detected waveform of the intensity detecting type GMR element in correspondence with a projected portion of a gear of FIG. 4A and a rectangular wave output after shaping the waveform of FIG. 4B, when the detected waveform or the threshold voltage Vta, Vtb is further shifted by the temperature drift, there poses a problem that as shown by a diagram of enlarging a vicinity of the detected waveform threshold of FIG. 5 (in temperature drift), the threshold voltage comes to the hem portion of the detected waveform (in temperature drift), pulse widths T1, T2, T3 of a rectangular waveform of FIG. 4B are provided with different values, that is, rectangular waves which differ by respective projected portions of the gear are outputted.